This present invention relates in general to metrology devices and in particular to correcting the polarization sensitivity of metrology devices.
Metrology devices using rotating polarizers typically suffer from polarization sensitivity. Polarization sensitivity is caused by components of the metrology device, e.g., beam splitters and spectrographs, being sensitive to the polarized state of light. As the polarizer changes the polarization state of the beam of light, the polarization sensitive components will affect the beam of light in different ways. Consequently, the polarization sensitivity of the components of the metrology tool can affect the measurement of a sample.
Thus, it is desirable to correct or avoid polarization sensitivity in a metrology tool. A common technique to avoid polarization sensitivity, often used in ellipsometers, is the use of a polarization scrambler or depolarizing prism in front of the polarization sensitive components. Unfortunately, the polarization scrambler and depolarizing prism significantly reduce the amount of light approaching the detector. Further, polarization scramblers are bulky adding extra complications if the system is to be compact.
Accordingly, a procedure is needed to correct the polarization sensitivity in metrology tools to avoid the use of additional bulky components that reduce the amount of light received by the detector.
In accordance with an embodiment of the present invention, a method of correcting the polarization sensitivity of a metrology tool, such as a normal incidence polarized reflectance spectrometer or an ellipsometer, includes separating the angular dependence of the irradiance, e.g., the reference irradiance and/or back reflection irradiance, from the temporal dependence. The angular dependence of the metrology tool is modeled, e.g., using a Fourier expansion or any other appropriate manner. If a Fourier expansion is used, the Fourier coefficients are parameterized as a function of wavelength. The actual polarization dependent irradiance, e.g., the reference irradiance and/or back reflection irradiance, is then measured for the metrology tool for one angle of the rotatable polarizer. From the measured irradiance and the modeled angular dependence, the total irradiance of the metrology tool can be determined. The total irradiance is independent of the angle of the rotatable polarizer. With the total irradiance known, the polarization dependent irradiance, e.g., reference and/or back reflection irradiance, can then be determined for any desired angle of the rotatable polarizer using the total irradiance and the angular dependence of the metrology tool.